Subclass
Ammonoidea - Ammonites

Ammonites
are an extinct marine taxon (subclass Ammonoidea) in the Phylum
Mollusca and Class Cephalopoda. Their closest living relative is
probably the modern nautilus that they closely resemble. Ammonite
fossil shells are of particularly beautiful
spiral forms, except for some more uncommon forms without spirals
that are called heteromorphs.
The name ammonite derives from the organism’s resemblance
to a coiled ram’s horn, after the god Ammon (that was depicted
as a man with ram's horns).

The
ammonite’s shell contained a spiraling progression of ever
larger chambers divided by thin walls called septa. The animal
only occupied the last and largest chamber. A thin living tube
called a siphuncle passed through the septa, extending from the
ammonite's
body into the empty shell chambers. The ammonite secreted gas
into these shell chambers, enabling it to regulate the buoyancy
of the shell. As the ammonite grew, it added newer and larger
chambers toward the larger open end of the coil.

While
the majority of ammonites have a shell that is a flattened coil,
others have a shell that is partially uncoiled, partially coiled
and partially straight (as in Australiceras), nearly straight
(as in baculites and belemnites), or coiled helically - superficially
like that of a large gastropod (as in Turrilites and Bostrychoceras).
These partially-to-totally uncoiled forms appeared in the Lower
Cretaceous and are known as heteromorphs.

Ammonites
first appeared in the late Silurian
to early Devonian Periods (~400 million years ago) and became
extinct at the close of the Cretaceous along with the dinosaurs
(some 65 million years ago). They underwent repeated and large
radiations only to fall victim to several extinction events. Ammonites
were especially abundant in the Mesozoic
marine environment due to rapid diversification, leading to widespread
distribution, making them particularly useful to geologists and
paleontologists as index fossils for biostratigraphy. Only some
10% of species survived the Permian Extinction, and their ultimate
demise was alonside that of the dinosaurs in the K-T event that
ended the Cretaceous Period.

While
the majority of ammonites have a shell that is a flattened
coil, others have a shell that is partially uncoiled, partially
coiled and partially
straight (as in Australiceras), nearly straight (as in
baculites and belemnites), or coiled
helically - superficially like that of a large gastropod
(as in Turrilites and Bostrychoceras). These partially-to-totally
uncoiled forms appeared in the Lower Cretaceous and are known
as heteromorphs.